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Ozone Microsparge System Annual Status Report—Former Tosco(76)Station No 6981,Stockton,California March 3,2003 The other onsite wells have demonstrated similar downward trends after the first year of treatment Apart from data obtained on September 30, 2002, none of the monitoring wells have increased in concentration during the last year Data obtained from September 30, 2002 reported_elevated concentrations for several of the wells which have historically been below detection limits The data from the December 31, 2002, groundwater sampling event has reported that concentrations have returned to previous levels, indicating that the September 30, 2002, concentrations were anomalous CONCLUSIONS AND RECOMMENDATIONS The results of the concentration versus time charts show that hydrocarbon concentrations have generally declined or remained stable during the past year of remedial system operation The ozone sparge system has been successful in reducing petroleum concentrations in the groundwater at this site This includes TPHg and benzene,which are the compounds of primary concern The current ozone infection addresses the dissolved impact onsite and will also act as a barrier to mitigate any continuing migration of dissolved hydrocarbons offsite Treatment directly offsite is not feasible due to the proximity of Pacific Avenue, however the offsite well MW-15 can continue to be monitored to verify that contamination is not moving offsite • The ozone micros arges stem will continue to operate for at least the next six months, and p Y P monitoring wells MW-16 and VW-7 will continue to be sampled on a monthly basis A semi-annual status report will be prepared and submitted after the next six months of operation System Information Startup Date December 19, 2001 Months of Operation 13 Number of ozone infection points 13 (SP-1 through SP-13 onsite, but SP-1, SP-3, and SP-10 are currently not active) Periodic groundwater sampling MW-16, VW-7 i140124 08 5